This invention relates to thermal control paints, and, more particularly, to a white paint useful in spacecraft applications.
Spacecraft are subjected to a wide range of thermal environments during service. One side of the spacecraft may face free space, while the other side faces the sun. Heat is radiated into free space to cool the spacecraft, but the spacecraft can be heated intensively in direct sunlight.
Active and passive temperature control techniques are used to maintain the interior temperature of the spacecraft, which contains persons or sensitive instruments, within acceptable operating limits. Active temperature control usually involves machinery or electrical devices, such as electrical heaters, electrical coolers, and heat pipes. The present invention deals with passive temperature control, where no machinery or electrical devices are used.
An established approach to passive temperature control is the use of surface coatings, typically termed "paints", on the external surface of the spacecraft. A white paint, for example, has a low solar absorptance, while a black paint has a high solar absorptance. The selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature. The present invention deals with a white paint that is useful in spacecraft temperature control applications.
In most cases, the paint must dissipate electrostatic charges that develop on the external surface of the spacecraft, as well as provide passive thermal control. The charges would otherwise accumulate to cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the paint must be somewhat electrically conductive, with a surface resistivity on the order of about 10.sup.9 ohms per square or less.
A spacecraft paint, whatever its color, should exhibit additional characteristics for spacecraft applications. The paint should be stable during long-term service in a space environment. The paint is desirably moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.
There are white, electrostatic-dissipative paints known for spacecraft use. The best of the known paints using an inorganic potassium silicate binder typically has a solar absorptance of from about 0.13 to about 0.15, see U.S. Pat. No. 5,094,693, whose disclosure is incorporated by reference. No white, electrostatic-dissipative paints of lower solar absorptance are available. The lower the value of the solar absorptance, the lower the heating of the paint and thence the underlying substrate, in the intense heating of direct sunlight.
There is a need for an improved white thermal-control paint that is operable and stable in a space environment, which has a lower solar absorptance than available in existing paints, and which can manage electrostatic discharge (ESD). The present invention fulfills this need, and further provides related advantages.